1. Field of the Invention
The present invention relates to an apparatus for measuring particles size distribution in which the particle size distribution of sample particles is measured by utilizing a diffracting phenomenon or a scattering phenomenon brought about by irradiating dispersed particles with a light.
2. Description of Related Art
In an apparatus for measuring a particle size distribution by utilizing a diffracting phenomenon or a scattering phenomenon of light by particles, the intensity distribution of the diffracted light or scattered light is used. In such a measurement, a relationship between a diffracted angle or a scattered angle and an optical intensity is measured and the output signals are subjected to an operation based on a theory of Fraunhofer diffraction or Mie scattering to calculate the particle size distribution of the sample particles.
FIG. 5 is a diagram showing a measuring optical system of a conventional apparatus for measuring a particle size distribution. A sample cell 1 is a transparent vessel, in which a medium with sample particles dispersed therein is housed. The sample cell 1 is irradiated with parallel laser beams L from an optical system 3 composed of a laser beam source 2 and the like.
The laser beams L, diffracted or scattered, by sample particles within the sample cell 1, are received by a ring-shaped detector 5 through one of the collecting lenses 4a, 4b, 4c, which are selected to determine the particle size distribution of the sample particles from a measured optical intensity distribution of the laser beams L.
In the above-described apparatus, in order to focus the diffracted or scattered laser beams L into an image on the ring-like detector 5, a plurality of condenser lenses 4a, 4b, 4c, each different in focal distance, are provided. In a measurement of sample particles having smaller particle diameters exhibiting larger scattering angles, the condenser lens 4a having a short focal distance (f1) is used while the ring-shaped detector 5 is also shifted to a position farther from the condenser lens 4a. On the contrary, in the case where sample particles having larger particle diameters exhibiting smaller scattering angles, the condenser lens 4c having a long focal distance (f3) is used while the ring-shaped detector 5 is also shifted to a position close to the condensed lens 4c.
A plurality of condenser lenses 4a, 4b, 4c, different in focal distance, are exchangedly used in the above-described manner, so that the particles size distribution can be measured over a wide range of particle sizes, as shown in FIG. 6.
Referring to FIG. 6, a range A of particle sizes having smaller particle diameters is measured by the use of the condensed lens 4a having a short focal distance (f1), a range B of particle sizes having particle diameters larger than those of range A is measured by the use of the condenser lens 4b having a slightly longer focal distance (f2), and a range C of particle sizes having still larger particle diameters is measured by the used of the condenser lens 4c having a still longer focal distance (f3), respectively.
The above conventional apparatus for measuring a particle size distribution requires a construction so that the respective condenser lenses 4a, 4b, 4c are changed for each range of particle sizes corresponding to the respective condenser lenses 4a, 4b, 4c. Additionally, the ring-shaped detector 5 is shifted as occasion demands, for each individual range of particle sizes to conduct the measurement. A problem has occurred when the sample particles extend over a plurality of ranges of particle sizes because the optical system must be changed many times, and thus the particle size distribution may not be accurately measured.
In addition, samples to be measured are frequently different in the range of particle sizes, respectively, so that a problem has also occurred in the it is necessary to conduct the above-described changeover of the optical system for every different sample, and thus the measuring operation can be troublesome.